The present invention relates to methods and apparatus for heat treating metallic material, and more particularly the invention relates to a method and apparatus wherein a metallic material to be heat treated, e.g., a long material such as steel pipe, steel bar, shape steel, rail or the like is conveyed in its lengthwise direction at a stable speed, thereby preventing the occurrence of non-uniform heat treatment and ensuring improved heat treating efficiency.
In the past, where a long metallic material such as represented by large diameter steel pipe or the like is subjected to heating treatment over the entire length thereof by continuously conveying the material through a relatively short heating zone, it has been the usual practice to ensure that the material is fed at a constant speed as far as possible so as to prevent non-uniform heating of the parts of the material, that where the heating is effected by induction heating thus causing the ends of the material to tend to be underheated, preliminarily a dummy is joined to each end or excess length for cutting allowance is provided at each end so as to cut off the same after heat treatment, that straightener rolls are provided to remove any distortions produced in the material during the heat treatment, and so on.
In particular, where large diameter steel pipe or the like is heat treated with an induction heating coil, due to the fact that the length of the induction heating coil is extremely small as compared with the length of the metallic material to be heat treated, it is an essential requirement that the metallic material is moved through the coil at a constant speed at all times. In other words, if the travel speed of a metallic material through the coil differs for different positions of the material, this causes the heating time to differ for the different positions of the material with the result that even if the amount of heat supplied per unit time is made constant by the electric heating method, i.e., induction heating (even if the uniform heating of the metallic material in a plane normal to the direction of travel of the material is made easier), the metallic material cannot be heated uniformly in time and consequently the different parts of the material subjected to heating process will be heated to different temperatures, that is, the temperatures of these parts will not be uniform. Of course, this tendency becomes more marked with increase in the preset heating temperature of the coil.
It a metallic material to be subjected to heating treatment is exposed to different heating conditions locally (in the lengthwise direction), the following problems will be caused. In other words, firstly the mechanical properties of the heat treated material will not be the same throughout the material. Secondly, change of shape will be caused by the non-uniform heating. These phenomena will become more marked with increase in heating temperature and increase in the non-uniformity of heating due to the irregularity in the travel speed of the material, and these phenomena also become more marked with increase in the cooling rate for the cooling process following the heating process. As a result, where the material is heated to a relatively high temperature and then cooled rapidly as during a quenching treatment, these phenomena will be still more marked. Up to date, the following conveying methods have been used in connection with the heat treatment by induction heating of such metallic material as large diameter steel pipe.
(i) Roller conveyor method:
In this method, metal material, e.g., steel pipe is placed on a roller conveyor and the material is conveyed by means of one or a plurality of drive rolls. While this method is suited for fast feeding purposes, there is a disadvantage that in the case of a treatment, e.g., heat treatment where the feeding speed is low (several tens to several hundreds milli per minute), it is difficult to maintain the feeding speed constant with the result that the conveying speed is made unstable due to slip between the material and the roll surface and consequently the heat treatment is effected non-uniformly.
(ii) Pinch roll method:
In this method, metallic material such as steel pipe is conveyed while the material is being held between a pair of top and bottom pinch rolls or between the rolls of a plurality of such pinch roll units which are arranged at a spacing. While this method is advantageous over the first-mentioned method in that the feeding speed is maintained constant, in the case of a metallic material having a circular section, for example, even if the rotational speed of the pinch rolls is maintained constant, there is the difference between the peripheral speeds at the center and marginal portions of the rolls and a slip will be caused between the metallic material and the rolls at some circumferential points of the material. As a result, if the shape of the metallic material is changed even a bit, the contact points between the pinch rolls and the material will be changed, thus changing the feeding speed. On the other hand, where the material is moved by the pinch rolls consisting of straightening rolls, a long material is passed between at least one pair of caliber rolls to straighten the bends and at the same time the material is conveyed at a desired speed to the heating zone by the rotation of the straightening rolls. Thus, while the feeding speed can be made relatively constant as compared with the case where the material is conveyed by feeding rolls, if there is any weld bead on the material or the material has been deformed, when the material contacts with the caliber rolls at a different part thereof, the feeding speed of the rolls will be charged at and around that part, thus making the feeding speed unstable and thereby making it impossible to accomplish both the desired straightening and the stabilization of feeding speed simultaneously.
Where the material is conveyed by driving the material feeding rolls and/or the straightening rolls, it is necessary to make the driving of the large number of such rolls to conform with one another throughout a wide range of conveying conditions. For instance, where the feeding speed of 100 to 600 mm/min is required for feeding the material in one direction for quenching treatment and then the material is fed in the opposite direction at the speed of 50 to 300 mm/min for tempering treatment, the roll speeds have a wide range of 1:2 (50 to 600 mm/min) with the result that a drive system and the associated units must be provided and the driving system becomes extensive and complicated. The driving system will be made more extensive, if the fast feeding necessary for increasing the efficiency of non-treating feeding and the reverse feeding of material is additionally required.
Further, since no material supporting rolls or the like are provided in the heating zone, with the conventional techniques the ends of the material bend downwardly while the material is introduced into and heated in the zone, and this constitutes a cause of non-uniform heating, bending or the like.
(iii) Car transport method:
This method is one in which a single car with a drive mechanism is run on the rails so as to convey a metallic material fixedly mounted in place on the car, and this method also has a problem of friction between the rails and the wheels of the car, thus causing a slip and thereby making the feeding unstable.
These are the disadvantages of the material conveying methods known in the art.
On the other hand, while methods are known in the art which are designed to prevent the ends of material from being heated non-uniformly, such as, one in which a dummy is joined by welding or the like to each end of the material so as to cut off the same after the heat treatment, and another in which an excess length is provided at each material end so as to cut off the same after the heat treatment, these methods also have the disadvantage of requiring additional labor and expenses and deteriorating the yield. Where a metallic material is heat treated by the conventional technique without joining any dummy to each material end, when the ends of the material move past the cooling unit, the cooling water, particularly the cooling water on the inner surface of the pipe will be discharged from the material ends and scattered in all directions, thus producing deteriorating effects on the electric equipment, etc., and also deteriorating the working environment.